In general, rotary furnace hearths are facilities heating, sintering, and reducing raw materials to recover high added value direct reduced iron. A rotary furnace hearth, as shown in FIG. 1 and FIG. 2(a), is comprised of a carriage 20 of a ring-shaped (donut-shaped) planar shape, an iron plate 6 and hearth refractory bed 7 carried on the carriage 20, and refractories 8, refractories 13, hearth materials 16, ceramic fiber blankets 27, etc. carried on the top surface of the hearth refractory bed 7. On the hearth material 16, raw material 19 comprised of iron ore or dust, sludge, scale, and other iron-making waste generated from iron-making plants is placed and heated to a high temperature state by burners or other heating means. The heated raw material 19 is rotated inside the furnace chamber 2 so as to recover direct reduced iron (DRI) from the raw material 19.
For this reason, the hearth structure comprised of the hearth refractory bed 7, refractories 8, side blocks 9 and 10, refractories 13, etc. is exposed to a high temperature along with the raw materials, so the hearth structure unavoidably expands under heat. Due to this, there is the problem that the furnace side walls 3 and 4 and the hearth structure rotating inside it frequently come into contact resulting in damage to the facilities and obstruction of normal carriage rotation. Therefore, to prevent damage to the facility and contact with the furnace side walls, it is necessary to provide the hearth structure of the rotary furnace hearth with a mechanism for absorbing the heat expansion.
In the past, as shown in FIG. 2(a), clearances (margins for expansion) 23 were provided between the refractories 13 laid between the side blocks 9 and 10 so as to absorb the refractory heat expansion, but in a rotary furnace hearth for recovering DRI, a powder or pellet hearth material 16 is laid on the top surface of the refractories 13 for preventing the DRI from melt bonding with the refractories 13, so there was the problem that hearth material 16 and the raw material 19 itself would drop inside the above heat expansion absorption clearances causing a loss of the heat expansion absorption function.
To solve this problem, Japanese Patent Publication (A) No. 2002-310564 discloses to prevent the hearth material 16 etc. from dropping into the clearances by the method of filling the clearances provided as the margins for expansion with ceramic fiber sheets or ceramic fiber blankets. Furthermore, to prevent the hearth material 16 or raw material 19 from entering the clearances 23, it may also be considered to lay ceramic fiber blankets 27 at the bottom surface of the hearth material 16.
However, the ceramic fiber sheets or ceramic fiber blankets etc. used as fillers are strongly compressed due to the heat expansion of the refractories under the high temperature atmosphere in actual operations and plastically deform as a result. After operation ends, the refractories are cooled and clearances form and do not return to the original state, so the hearth material 16 or DRI drop into and build up in the clearances formed. Further, the ceramic fiber blankets 27 laid at the bottom surface of the hearth material 16 shrink by the heat and are crushed by the hearth material 16 or raw material 19 to thereby break. It was therefore not possible to prevent the hearth material 16 and raw material 19 from dropping into the margins for expansion of the refractories over the long term.
In this way, in the method of filling the clearances provided as the margins for expansion with ceramic fiber sheets or ceramic fiber blankets etc. to absorb the heat expansion, there was the problem that it was only possible to obtain the heat expansion absorption function for the short time until the ceramic fiber blankets 27 broke.
In this way, with the conventional method, it was not possible to substantially permanently secure the heat expansion absorption clearances (margins for expansion) 23 along with actual operation. As shown in FIG. 2(b), it was not possible to completely avoid the heat expansion force of the refractories 13 from pushing apart the side blocks 9 and 10 at the two sides and the contact of the side blocks 9 built at the inner circumference end of the hearth refractory bed 7 and the inner circumference side furnace side walls 3 and contact of the side blocks 10 built at the outer circumference end of the hearth refractory bed 7 and the outer circumference side furnace side walls 4. That is, there was a structural limit in the conventional method of providing clearances (margins of expansion) 23 between the refractories laid between the side blocks 9 and 10 to absorb the heat expansion when laying the hearth material 16 on the top surface of the refractories 13.